One of the big knocks against the M-16/M-4 family is that it uses a system called “direct impingement” to cycle the weapon. Gas is tapped by a small hole under the front sight, directed via a stainless steel tube, and then strikes the bolt and bolt carrier directly. This causes carbon to build up rapidly and the weapon needs frequent cleaning and judicious use of lubrication. Most semi-automatic and automatic rifles use a piston system, where the gas instead strikes a piston that drives a rod connected to the bolt to cycle the weapon. While this reduces carbon buildup, it does mean more parts and weight.

XBradTC on why the Army is looking into an M-4 replacement. politics is one factor that delayed the M-4 replacement. Colt pitched a fit when the Army Times ran it’s story about the HK-416.

But now the Army is taking a look at retiring the black rifle. The potential replacements today are:

The XM-8

The HK-416, basically it’s just an M-16 with a gas piston instead of a gas tube.

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